FEG stands for Fegyver es Gepgyar (Hungary) The model GPM-01 (if this is the model you are asking about) is worth: 100%=$425, 90%=$315, 60%=$165 The GPM-01 is the Daisy Model 91 (imported by Daisy in 1991 to 1997.)
see the link below for the Owners manual.
It was designed for BB's .175 or .177
I need to know the condition of the pistol to give you an estimate of value. It was made between 1971-80
It won't fit in the pistol.
Benjamin model EB22. Crosman model 2240. Benjamin Marauder Air or C02 Pistol. All 3 of these are .22 cal C02 air pistols, There are several others but they are spring or pneumatic not C02 systems
Umarex USA makes the H&K pistol. I suggest you contact them. See the link below
CO2
You can buy any gun in green gas, CO2, or electric. You just have to look for them....
CO2 air pistols can be purchased from sporting goods stores, online retailers, gun shops, and specialty stores that sell air guns. Make sure to check local laws and regulations regarding the purchase and use of air pistols in your area.
With enough money and a talented craftsman anything is possible. Any competent engineer will be able to machine an offset converter to allow the use of a regulator and air cylinder from a pistol such as the Rohm Twinmaster.
It means it's not powered by electric or CO2 if it is spring powered you have to cock it after each shot
Try a large well stocked air gun shop.